Some image sensors, typically CCD image sensors utilizing CCD solid image pickup elements, perform a logarithmic conversion on the brightness signals they receive to expand their dynamic ranges.
This type of image sensors, called logarithmic sensors, (hereinafter referred to as LOG image sensors) have very wide dynamic ranges capable of covering dark night sceneries and bright snowy sceneries as well, as depicted by a brightness-output characteristic curve (solid line) shown in FIG. 1. Comparing the brightness-output characteristic of such LOG image sensor with that of a conventional CCD sensor shown by a dotted curve, it is seen that the former sensor has a much wider dynamic range than the latter.
However, since the output voltage of a LOG image sensor represents a logarithmically converted brightness of an object, image data output therefrom are distributed within a narrow voltage band, as indicated by ΔV in FIG. 1, so that the image has a poor contrast when displayed as it is.
However, inversion of the logarithmically converted image data back to their original linear image data requires a complex data processing.